C.J. Van Tyne

Failure During Sheared Edge Stretching

Failure during sheared edge stretching of sheet steels is a serious concern, especially in advanced high-strength steel (AHSS) grades. The shearing process produces a shear face and a zone of deformation behind the shear face, which is the shear-affected zone (SAZ). A failure during sheared edge stretching depends on prior deformation in the sheet, the shearing process, and the subsequent strain path in the SAZ during stretching. Data from laboratory hole expansion tests and hole extrusion tests for multiple lots of fourteen grades of steel were analyzed. The forming limit curve (FLC), regression equations, measurement uncertainty calculations, and difference calculations were used in the analyses. From these analyses, an assessment of the primary factors that contribute to the fracture during sheared edge stretching was made. It was found that the forming limit strain with consideration of strain path in the SAZ is a major factor that contributes to the failure of a sheared edge during stretching. Although metallurgical factors are important, they appear to play a somewhat lesser role.

Analytical prediction of springback based on residual differential strain during sheet metal bending

As the springback of sheet metal during unloading may cause deviation from a desired shape, accurately predicting springback is essential for the design of sheet stamping operations. Finite-element models have not been successful in predicting springback; hence there is a need for analytical models to make such predictions. In this study, a model based on differential strains after relief from the maximum bending stress is derived for six different deformation patterns in order to predict springback analytically. The springback for each deformation pattern is estimated by the residual differential strains between outer and inner surfaces after elastic recovery. Each of the six deformation patterns has a valid region of applicability, based on elastic modulus, yield strength, applied tension, and bending geometry. Analytical equations for the springback of the sheet deformed under these six deformation patterns are derived. Traditional analytical models for springback prediction have been based on elastic unloading from a bending moment. Traditional models also require the knowledge of the stress distribution through the thickness of the sheet, whereas the residual differential strain model only requires the stress state on the outer and inner surfaces of the sheet. In order to compare the residual differential strain model with the traditional bending moment model, a bending moment model is derived for the same exact deformation patterns. Results from the two models are compared for various materials.

Angle of contact between sheet and die during stretch–bend deformation as determined on the bending-under-tension friction test system

Abstract Laboratory tests, designed to assess the frictional behavior and formability of sheet metal, rely on direct measurement of pulling and contact forces for friction evaluation. The bending-under-tension friction test subjects a strip to bending as it slides over a cylindrical die surface and can evaluate sheet forming behavior during stretch–bend deformation. To gain a better understanding of sheet–die interaction, the bending and unbending response of the sheet as it contacts the cylindrical die was investigated. The existence of bending and unbending pressure peaks as well as the resulting extent of the angle of contact between the sheet and the die was observed experimentally using a Kynar® piezoelectric sensor. Direct evidence of pressure peaks during stretch–bend deformation of sheet steels is presented. The extent of the contact angle is less than the geometrical angle-of-wrap and increases with strip tension. The experimental findings for contact angles compare favorably to the predicted results from a simplified vector analysis of strip tension and strip-bending resistance.

Review of the Shearing Process for Sheet Steels and Its Effect on Sheared-Edge Stretching

Failure in sheared-edge stretching often limits the use of advanced high-strength steel sheets in automotive applications. The present study analyzes data in the literature from laboratory experiments on both the shearing process and the characteristics of sheared edges. Shearing produces a surface with regions of rollover, burnish, fracture, and burr. The effect of clearance and tensile strength on the shear face characteristics is quantified. Higher strength, lower ductility steels exhibit an increase in percent fracture region. The shearing process also creates a zone of deformation adjacent to the shear face called the shear-affected zone (SAZ). From an analysis of data in the literature, it is concluded that deformation in the SAZ is the dominant factor in controlling failure during sheared-edge stretching. The characteristics of the shear face are generally important for failures during sheared-edge stretching only as there is a correlation between the characteristics of the shear face and the characteristics of the SAZ. The effect of the shear burr on shear-edge stretching is also related to a correlation with the characteristics of the SAZ. In reviewing the literature, many shearing variables that could affect sheared-edge stretching limits are not identified or if identified, not quantified. It is likely that some of these variables could affect subsequent sheared-edge stretching limits.

Validation via FEM and plasticine modeling of upper bound criteria of a process-induced side-surface defect in forgings

Abstract Finite element analysis and plasticine modeling has been performed for an axisymmetric forging with a double ram action to validate the results of upper bound analysis which provided the geometric and processing conditions that can be used to avoid the development of unacceptable side-surface cracks. The finite element analysis, by using NIKE2D, an implicit finite deformation FEM program, has provided detailed information about the stress- and strain-states that the workpiece experiences and shows that large axial strains develop in the mid-thickness region at small workpiece thicknesses, which correlates to the conditions of surface-crack formation as predicted by the upper bound analysis. Surface cracks obtained in an FEM analysis of two half-billet workpieces have shown excellent agreement with the trends developed through the upper bound analysis. The physical modeling experiments with plasticine have provided further validation of both the upper bound model and the criteria curves for the prevention of side-surface defects. The process of side-surface cracking in a double action axisymmetric forging process was analyzed by the upper bound method. The results of this analysis are presented elsewhere (Y.H. Moon, C.J. Van Tyne, W.A. Gordon, J. Mater. Process. Technol., WTRA, pp. 412, 413). The prime objective of the present paper is to provide validation of the cracking criteria that was developed, this validation being done by both the numerical finite element method and by the use of plasticine modeling material with plexiglass tooling in laboratory experiments.

Deformation analysis for cold rolling of Al-Cu double layered sheet by physical modeling and finite element method

The deformation characteristics of Al-Cu double layered sheet during rolling with various process parameters were studied by both a physical modeling technique and the finite element method. Physical modeling and the finite element method are complementary, due to their different advantages and limitations. Physical modeling simulates metal forming operations by using a model workpiece under conditions similar to those in actual production. The deformation characteristics of double layered sheet during rolling were also simulated using a commercial finite element code, FORGE™. The effects of process parameters, such as total reduction ratio, initial thickness ratio and differential speed ratio on the rolling characteristics were the primary focus of the investigation. In addition, an analytical model for double layered sheet rolling is also proposed with the use of a force-thickness diagram. From the results, the effect of the process parameters on the rolling of the Al-Cu double layered sheet has been determined.

Effect of a Strain-Hardening Rate at Uniform Elongation on Sheared Edge Stretching

Edge failure during stretching of sheared edges limits the use of sheet steels in a number of product applications. The shearing process causes a highly strained region adjacent to the shear face, called the shear-affected zone. In the present study, the strain-hardening rate at uniform elongation, Z, is used as an empirical measure of cohesive strength at the interface of the various phases in steel microstructures. The higher the value of Z, the lower the macro strain when voids begin to form that lead to decohesion of the interface and subsequent failure. The data from four different studies are used to show that the true circumferential strain at failure in a hole expansion is a direct function of Z for most microstructural conditions. Sheet steels that exhibit better performance than that which would be expected for their Z values have one or more of the following characteristics—an increase in ferrite strength, lower carbon martensite in DP steels, or TRIP steels. A hot-rolled ferrite/pearlite microstructure is the only case of decreased true circumferential strain at failure for a given value of Z.

Evolution of die surfaces during repeated stretch-bend sheet steel deformation

Abstract The friction and wear characteristics of several die materials used for sheet metal forming have been investigated. The study examines five die materials tested in conjunction with three commercially produced automotive sheet steels. The die materials were: A2 tool steel, weld overlay, tungsten carbide, physical vapor deposited titanium nitride and transformation toughened zirconia. The three steels were: a low carbon uncoated aluminum killed drawing quality (AKDQ) sheet, a zinc-coated galvannealed sheet and a hot dip galvanized sheet. Each die material was tested using a repeated stretch-bend deformation with each steel sheet in order to examine the initial stages of die wear. The changes in the die surface profile were examined after approximately 140 tests and compared to the profile before testing. The tungsten carbide die material exhibited the least amount of change. Even though it was the smoothest before testing, the zirconia die material showed the most significant change, especially when tested with the two zinc-coated sheet steels.

Evolution of the tribological characteristics of several forming die materials

Abstract The purpose of the present study is to investigate the friction and wear characteristics of forming die materials. Three automotive sheet steels and five die materials were tested. The steels were aluminum-killed drawing quality (AKDQ) uncoated sheet, galvannealed sheet and hot-dipped galvanized sheet. The die materials were A2 tool steel, weld overlay, tungsten carbide, titanium nitride coated and zirconia. Bending-under-tension friction tests were performed to evaluate frictional characteristics and to examine the initial stages of wear. The sheet materials exhibited frictional characteristics that are relatively independent of the die surface. The galvannealed sheet had the greatest overall coefficient of friction, the hot-dipped galvanized coated sheet had the lowest and the AKDQ sheet had a value between that of the two coated sheets. Of the die materials tested, the tungsten carbide and the zirconia die materials exhibited the lowest coefficients of friction. The A2 tool steel and the tungsten carbide die materials maintained a relatively constant coefficient of friction throughout testing. Overall, the tungsten carbide die material had the best performance with respect to the lowest coefficient of friction and highest resistance to wear.

Failure during Sheared Edge Stretching of Dual-Phase Steels

The use of dual-phase steels has been limited in a number of applications, due to failure during sheared edge stretching. Previous investigations have studied the properties of dual-phase steels, especially regarding the mechanical properties of the individual phases or constituents, the strain partitioning to the microconstituents during loading, and the decohesion at the interface during loading. On the basis of the literature review, a hypothesis is developed in which failure in sheared edge stretching is the result of a sequence of events. Cracking first develops in the hard constituent, cracks grow in the interface between the hard constituent and ferrite, and relative movement of ferrite relative to the hard constituent increases the rate of cracking. In the present study, a single steel was heat treated to produce different amounts of hard constituent within the ferrite matrix in order to better understand the behavior of dual-phase steels during sheared edge stretching. The results of the study are consistent with the proposed hypothesis. It was found that in contrast to other studies, increased strength of the hard constituent retards crack initiation. Crack growth increased with increasing surface area of hard constituent–ferrite interfaces and increasing movement of ferrite relative to the hard constituent.